There are a number of techniques that enable drilling of subsea wells. In conventional re-entry drilling of a subsea well, in order to access a subsea well a drilling rig is positioned above the well, a rigid riser and blowout preventor system are lowered into the sea and attached to a subsea wellhead. A drill string is lowered into the riser and well using the drilling rig hoisting system mounted on the drilling rig and then drilling can occur. Generally the cuttings produced during the drilling are brought to the surface using a direct continuous circulation process whereby drilling fluid is pumped down the drill string, comes out of the drill bit and returns to the drilling facility at the water surface through the annular space between the drill string and borehole then the annular space between the drill string and casing and finally the annular space between the drill string and the riser. A mud treatment system is then used to separate the drill cuttings from the drilling fluid which is then pumped down the drill string again, this is a continuous circulation process. However such conventional drilling operations require the use of expensive drill ships or floating rigs.
The present invention aims to provide a system to allow further drilling of existing subsea wellbores in an underbalanced mode from a surface vessel, such as a light intervention vessel.